While a cavity waveguide has been practically used as a low-loss transmission line for microwaves or millimeter waves, it involves difficulties in application to small-size electronic devices, such as portable communication terminals, due to inevitable increase in size and weight. In this connection, it is contemplated to utilize a dielectric waveguide which is prepared by forming a conductive film on a surface of a dielectric material. The dielectric waveguide has the advantage of effectively shortening the wavelength of an electromagnetic wave through its dielectric transmission line and eliminating the need for using a thick metal wall so as to facilitate downsizing and weight reduction thereof. This means that the dielectric waveguide has the potential to be mounted on commonly used printed circuit boards. Thus, the dielectric waveguide is regarded as one of noteworthy transmission lines for a small-size electronic component circuit usable in a high-frequency band, and various development efforts are being made toward its practical use.
Generally, an electromagnetic wave is transmitted through a microstrip line formed on the printed circuit board and a dielectric waveguide in different propagation modes. Therefore, in cases where the dielectric waveguide is used in such a manner that it is mounted on the printed circuit board and connected to the microstrip line, it is required to provide a mode conversion mechanism for converting one propagation mode in the microstrip line to the other propagation mode in the dielectric waveguide (see, for example, Japanese Parent Laid-Open Publication No. 2002-135003). This mode conversion mechanism is desired to be structurally simple and operable in a wide-frequency band. Further, if a dielectric waveguide is connected directly onto a microstrip line for use in a high-frequency band of 20 GHz or more, even a slight displacement therebetween will be highly likely to cause significant change in mode conversion characteristics and deterioration in practicality.